Application of 3D heteroatom-doped graphene in adsorptive removal of water pollutants: Review on hydrothermal synthesis and its influencing factors

Graphene in its various forms including three-dimensional (3D) structure has recently garnered considerable recognition due to its unique physicochemical and mechanical attributes. Specifically, its ultra-large surface area, tunable surface functionalities, abundant adsorption mechanisms and enhanced practical handling have endowed graphene as a prospective adsorbent for environmental remediation. Heteroatom doping offers a promising approach for improving the adsorption properties of the nanomaterial. Thus, this review examines the potential of heteroatom-graphene adsorption systems in removing harmful aqueous pollutants. Here, the adsorption kinetic, equilibrium and thermodynamics of heteroatom-doped graphene structures are discussed. The involvement of adsorption mechanisms such as π-π stacking, hydrogen bonding, electron donor-acceptor, hydrophobic and electrostatic interactions in sequestrating the pollutants are also reviewed. Additionally, the state-of-the-art assessment on the enhancement of adsorption properties induced by heteroatom doping such as new functional groups with changeable charge, expandable interlayer spacing, larger surface area and wettability is presented. Thereafter, the review focuses on hydrothermal synthesis of heteroatom-doped 3D graphene, a facile and scalable method which has received very limited attention to date. The hydrothermal governing parameters are discussed and evaluated with respect to the physicochemical and adsorption properties of the doped graphene. The utilization of density functional theory calculations to model 3D heteroatom-doped graphene systems for wastewater remediation is also studied. Finally, the review summarizes the current developments and identifies the challenges associated with hydrothermally synthesized 3D heteroatom-doped graphene for future work. Overall, this study provides valuable insights into the use of heteroatom-doped graphene adsorbents for aqueous pollutants removal as well as the strengths of heteroatom doping and hydrothermal synthesis in enhancing their adsorption efficacy.